1. Field of the Invention
The present invention relates to a proton conductive electrolyte membrane and a method for producing proton conductive electrolyte membrane, and particularly relates to a fuel cell using a proton conductive electrolyte membrane as an electrolyte, which is an electrolyte membrane for a direct methanol solid polymer fuel cell.
2. Description of Related Art
A fuel cell is a power generation device generating electricity by reacting hydrogen and oxygen. Fuel cells draw attention as an energy-saving technology dealing with global environmental problems such as the warming and the ozone layer depletion because of having an excellent character that only water is formed by the generating reaction.
There are 4 types of fuel cells, a solid polymer fuel cell, a phosphoric acid fuel cell, a molten carbonate fuel cell and a solid oxide fuel cell. Among them, the solid polymer fuel cell has advantages of a low operating temperature and a solid (polymer membrane) electrolyte. The solid polymer fuel cell is classified broadly into 2 types composed of a reformer type, which change methanol into hydrogen by using a reformer, and a direct type (DMFC: Direct Methanol Polymer Fuel Cell), which uses methanol directly without using a reformer. Because no reformer is necessary for DMFCs, DMFCs can be made smaller and lighter, and have been expected to be put into practical use as a cell or a special battery of a PDA (Personal Digital Assistance) or the like for a coming ubiquitous society.
Main components of a solid polymer fuel cell are electrode catalyst, electrolyte and a separator. A polymer proton conductive electrolyte membrane is used as the electrolyte. While proton conductive electrolyte membranes are used for application such as an ion-exchange membrane or a temperature sensor, proton conductive electrolyte membranes get attention in application as electrolyte in a solid polymer fuel cell in recent years. For example, a fluororesin membrane containing a sulfonic group, which is represented by Nafion® of DuPont Corp., has been studied in use as electrolyte in a portable fuel cell.
These fluororesin proton conductive membranes known heretofore have a defect of large methanol permeability. Development of a membrane with high proton conductivity and low methanol permeability is essential for achieving the practical use of a proton conductive membrane in new application of a solid polymer fuel cell such as a DMFC. In particular, reduction of the thickness is also necessary for improving performance as a DMFC, and also the physical strength of the membrane is required.
There is proposed various methods of impregnating a porous membrane having vacancies with a proton conductive polymer to obtain a proton conductive membrane.
JP-Tokukaihei-10-312815A discloses a composite membrane in which an ion conducting polymer is embedded within a porous substrate formed of randomly oriented individual fibres for the purpose of providing a novel composite ion exchange membrane with improved dimensional stability and handlability, and in which the ionic conductivity and reactant gas cross-over have not been compromised compared to a conventional unreinforced ion exchange membrane of the same polymer and comparable thickness.
WO00/54351 pamphlet discloses an electrolyte membrane in which pores of porous substrate substantially not swelling with methanol and water are filled with a polymer having proton conductivity for the purpose of providing an electrolyte membrane suppressing methanol permeability (crossover) as much as possible and working in a high-temperature (about 130° C. or more) condition. An inorganic material such as ceramic, glass and alumina or a heat-resisting polymer such as polytetrafluoroethylene and polyimide is used as the porous substrate. It is described that preferably the porosity of the porous substrate is 10 to 95%, the average vacancy size is 0.001 to 100 μm and the thickness is in the order of magnitude of several microns.
JP-Tokukai-2002-83514A discloses a proton conductive membrane in which a polymer having a phosphate group, a phosphonate group or a phosphinate group in the side chain is supported in vacancies of a porous membrane for the purpose of a proton conductive membrane having durability and mechanical strength. Ultrahigh molecular weight polyorefin resin and fluororesin are taken as the porous membrane. It is described that preferably the porosity of the porous membrane is 30 to 85%, the average vacancy size is 0.005 to 10 μm and the thickness is 5 to 500 μm.
For making a proton conductive electrolyte membrane capable of practical use as electrolyte of a solid polymer fuel cell, it is important factors that at least proton conductivity is sufficiently high, methanol permeability is sufficiently low and physical strength is sufficient for reducing the thickness adequately. Additionally, it is desired that mass production of proton conductive electrolyte membranes with industrially stable quality becomes possible.
Since the development described in JP-Tokukaihei-10-312815A has a tendency for distribution of holes in a porous support to be uneven, it is difficult to obtain products having sufficient performance in terms of proton conductivity. The development described in WO00/54351 pamphlet is focused on improvement of methanol permeability and working in a high temperature not less than about 130° C. The development described in JP-Tokukai-2002-83514A is focused on durability and mechanical strength. Referring to WO00/54351 pamphlet and JP-Tokukai-2002-83514A cannot make it possible to obtain a proton conductive electrolyte membrane satisfying at least 3 qualities that the proton conductivity is sufficiently high, the methanol permeability is sufficiently low and the physical strength is sufficient for reducing the thickness adequately.